1. Field of the Invention
This invention relates to seismic geophysical surveying. More particularly it relates to a type of explosive charge known as a linear charge, a constant detonation-velocity charge, or a velocity-matching charge, in which the average velocity of detonation of the charge along its length, is substantially the same over equal increments of length of the charge.
Still more particularly it relates to a multiunit, serially-connected charge, having improved means for detonating from one unit to the next, and including the provision of synchronously detonating booster explosive units, as the detonation wave passes the joints between each of the units.
Still more particularly it relates to an improved type of separate booster explosive charge unit, for insertion at multiple spaced points, into a linear explosive charge.
2. Description of the Prior Art
In seismic geophysical surveying, wherein artificial seismic waves are created by detonating an explosive charge in the earth, and the resulting elastic or seismic waves are received by geophones, or other sensors, variously disposed on the earth's surface, it has become generally recognized that a long continuous explosive charge, or an array of many small point charges properly spaced and timed, such as to match the timing to the advancing of a seismic wave front through the adjacent earth medium, offers substantial advantages. Such elongated charges, or arrays, have directional properties, in that the created seismic waves travel in a preferred direction with maximum strength, and at the same time cause less unwanted disturbances travelling in other than the preferred direction.
In the prior art an example of the constant detonation velocity charge is described in U.S. Pat. No. 3,150,590 of Daniel Silverman. Use of this type of charge has been made in the geophysical industry where its particular advantages of directivity of energy delivery, and higher frequency content of the seismic energy, have proved useful in the solution of particular seismic problems.
However, a number of deficiencies and disadvantages of this particular design have been found, including high cost and high weight. A less expensive version of this design utilized a paper tube, in place of the wood stick. But this had other more serious disadvantages. In particular, the charges lacked capability of submersion in water in the shotholes for long periods. More serious, however, was the occurrence of a great number of misfires; that is, failure to detonate from one explosive unit to the next.
In the use of constant velocity charges, there is a limit to the total weight of explosive that can be used on a charge of a given length. Although a given weight of constant velocity charge, because of its directivity, is equivalent, for seismic recording purposes, to several times that weight of concentrated point charge, sometimes additional explosive weight is needed. This can be provided by inserting (preferably) into each of the junctions between adjacent sticks of the linear charge a separate booster explosive charge unit, which is synchronously detonated by the passage of the detonation wave along the explosive card. These separate booster explosive charge units form an important part of this invention.
BRIEF SUMMARY OF THE INVENTION
It is a primary object of this invention to provide a long, velocity-matching explosive charge, that will withstand longtime submersion in water without deterioration to the point of possible misfires.
It is a further object of this invention to provide a long, velocity-matching explosive charge made of many interconnected serial units that has reduced probability of misfires.
It is still a further object of this invention to provide a long velocity-matching explosive charge, the total charge weight of which can be augmented by the addition of concentrated explosive charges at each of the junctions between serially connected units.
It is a still further object of this invention to provide such concentrated charge units in two parts, a container part, without explosive material, and an explosive part, which is loaded into the container part just prior to insertion into the linear charge and into the shot hole.
These and other objects are realized and the limitations of the prior art are overcome in this invention by providing a velocity-matching charge which comprises a long, linear explosive charge having a plurality of serially connected units, or sticks. Each of the sticks is substantially identical, having the same length, the same coupling units on each end, the same weight per unit length of explosive cord, and the same length of explosive cord, thus providing the same time interval for the detonation wave to progress from one end of the stick to the other.
Each stick is made of plastic tubing, with threaded couplings of molded plastic. A hole is drilled at a selected distance from each end, through the wall of the stick. A selected length of explosive cord is wrapped in a helix, on the outer surface of the stick between the two holes. The turns of the helix are equally spaced, and the spacing is designed, in conjunction with the known velocity of detonation of the explosive cord, to provide a selected velocity of detonation from one end of the charge to the other. The two ends of the helix are inserted, one into each of the holes, to extend axially out of the couplings on each end.
For convenience, the end of the stick having the female coupling is considered the first end, and the cord is detonated at this end. The detonation then proceeds through the helix to the second end, with the male coupling.
A selected length of a thick-walled elastic tube that can be made of foamed rubber or platic, or equivalent, is positioned inside the first end of the stick, with the projecting end of the explosive cord passing axially up through the tube. The inner diameter of the tube is less than twice the outer diameter of the explosive cord. Thus, when the cord projecting from the second end of another stick is inserted into the tube in the process of joining two sticks, the elastic tube is stretched, and it provides compression along its full length, forcing the two ends of explosive cord into side-by-side intimate contact. This pressurized contact facilitates cross-detonation between the cords, and so prevents misfires.
Separate explosive charge units are provided, which are constructed as slender elongated containers, with appropriate female and male connectors at the first and second ends, respectively, of the unit. These connectors are identical to those on the separate sticks, and a container can be inserted in each of the junctions, or couplings, between consecutive sticks of the linear charge.
The explosive material can be inserted into the containers just prior to the time the unit is inserted into the linear charge, and the linear charge inserted into the shothole.
One embodiment of the invention involves a solid explosive material, of selected composition, which is formed in the shape of a slender cylinder having an axial opening therethrough. Detonation of the explosive material inside the container is by means of the end of the explosive cord extending from the second, or male, coupling of the stick. The explosive cord is inserted into the top of the axial opening in the charge.
A stub length of explosive cord is also inserted up into the bottom end of the axial opening of the explosive charge, so that it projects out of the male coupling of the charge unit, and can be inserted into the female coupling of the next stick.